1. Field of the Invention
The present invention relates to active matrix displays comprising active devices for controlling pixel units to control the display. In particular, the present invention relates to an active matrix organic electroluminescence (EL) display comprising organic EL devices serving as luminescent devices, or electro-optic devices, for the pixel units.
2. Description of the Related Art
Organic EL displays using organic EL devices as luminescent devices, each included in a pixel unit, have recently been thought of for flat-panel displays. Since the organic EL devices exhibit a luminance of hundreds to tens of thousands of cd/m2, an organic EL display using the organic EL devices as luminescent devices promise a flat panel display in the next generation.
The organic EL display may be driven by a passive matrix system or an active matrix system. The passive matrix system reduces an emitting period of pixels as the scanning lines increase in number. In order to achieve an upsized fine display, therefore, requires each organic device to instantaneously emit light with a high luminance. On the other hand, the active matrix allows the pixels to keep emitting over a frame, thus facilitating an upsized, fine display.
An organic EL device has a structure in which an organic layer including a luminescent layer lies between a transparent anode and a metal cathode. In an active matrix organic display, thin-film transistors (TFTs) are generally used as active devices for driving the organic EL devices. Pixel-driving circuits (hereinafter referred to as pixel circuits) including the TFTs are formed in a layer, and incorporated with organic EL devices, which are formed in another layer, corresponding thereto to form pixel units.
Specifically, the pixel circuits including the TFTs are prepared on a glass substrate to form a circuitry layer. A planarized layer is formed on the circuitry layer, and contacts for electrically connecting the pixel circuits with the organic EL devices are provided in the planarizing layer. Organic EL devices, in which the organic layer including the luminescent layer lies between two electrodes, are formed on the planarized layer to form a device layer. The organic layer generally has a very small thickness of a few micrometers or less, for example, a thickness of tens to hundreds of nanometers, and therefore the flatness of the lower electrode, on which the organic layer is formed, is extremely important.
The circuitry layer, in which pixel circuits including TFTs are arrayed according to pixels, is planarized by forming the planarizing layer. However, the formation of the contact causes an inevitable roughness or step of a few micrometers, thereby degrading the flatness of the lower electrode. As a result, shortcircuiting is exhibited between the upper and the lower electrodes. Also, thickness variation of the organic layer, which concentrates an electric field and deteriorate the device, can be exhibited in the surface direction.